Secondary batteries (storage batteries) such as lithium ion batteries and nickel hydride batteries have recently gained importance as power sources for installation on vehicles or power sources for personal computers and portable terminals. In particular, lightweight lithium ion batteries that make it possible to obtain a high energy density are expected to be advantageously used as high-power power sources for installation on vehicles. Examples of typical structures of such secondary batteries include batteries of sealed structure (sealed batteries) in which a case accommodating an electrode body and an electrolytic solution is sealed.
However, when a defective battery is present or an erroneous operation is performed due to failure of a charging device when the batteries of such kind are charged, an unusually high current can flow in the battery and the battery can be overcharged. When battery abnormality such as overcharging occurs, gas is generated inside the sealed battery case, the internal pressure in the case rises, and the battery bulges under the effect of such abnormal internal pressure (gas pressure). Under certain circumstances, the case can be ruptured. As the conventional measure against such abnormality, a battery structure has been suggested that is provided with a current cut-off valve that cuts off the electric current and releases the internal pressure when the internal pressure in the sealed battery case rises abnormally.
For example, Patent Documents 1 and 2 listed below disclose sealed batteries including a current cut-off valve connected to a linking portion (connecting member) of an electrode body by a rupturable metal foil that is ruptured under the effect of abnormal internal pressure, and when an abnormal pressure acts upon the current cut-off valve, the rupturable metallic foil is broken, the current cut-off valve is separated from the linking portion (connecting member) and the electric current is cut off. Patent Document 3 describes a structural example of a current cut-off valve representing the related art.
Patent Document 1: Japanese Patent Application Laid-Open No. H10-241653.
Patent Document 2: Japanese Patent Application Laid-Open No. 2007-227283.
Patent Document 3: Japanese Patent Application Laid-Open No. H8-115714.